1. Field of the Invention
The present invention relates in general to communication systems such as electronic exchanges and the like, and more particularly to speech path switching control apparatus and a method in which a disabling function of an echo canceller required by a CCITT recommendation is effectively performed and maintenance of speech is made possible even upon occurrence of a fault in echo cancellation.
2. Description of the Prior Art
Generally, in a long-distance telephone network or a digital mobile communication network, a delay is in excess of several tens ms due to a transfer distance, voice data processing, etc.
Two wires are used between a public switched telephone network and an exchange to perform bidirectional speech. Four wires are used in a digital exchange or telephone network to perform the bidirectional speech. For this reason, a 2/4 wire converter is used in a subscriber interface circuit of the public switched telephone network. In this case, an echo is generated due to an impedance mismatch of the subscriber interface circuit.
In the digital mobile communication network, voice coding is performed for voice data to reduce a using band by reason of the effectiveness of radio channels. In this case, a considerable processing delay occurs differently from a PCM manner or an ADPCM manner in the public switched telephone network.
Namely, when the digital mobile communication network is connected with the public switched telephone network, an echo with a considerable delay (e.g., greater than 100 ms) is generated. As a result, this echo must be canceled.
In the case where the digital mobile communication network is connected with the public switched telephone network, a part of voice data from a mobile station is reflected back to the mobile station as an echo by the 2/4 wire converter in the subscriber interface circuit. As a result, an echo canceller is built in a PCM trunk line between the digital mobile communication network and the public switched telephone network to cancel the echo. When PCM voice data from the mobile station is transmitted to the public switched telephone network, the echo canceller stores the PCM voice data and applies the stored PCM voice data to its digital filter which has the same characteristic as that of an impulse response of an echo path resulting from the delay through the 2/4 wire converter in the subscriber interface circuit. Then, the resultant output from the digital filter is subtracted from the echo reflected from the public switched telephone network. Therefore, a subscriber accommodated by the mobile station is not influenced by the echo.
Since such a conventional echo canceller is built in a T1 or E1 truck line separately from a mobile switching center, it is impossible to use maintenance and repair functions of the mobile switching center. For this reason, it is difficult for maintenance and repair of the echo canceller to be done.
In the case where the echo canceller must not be operated, such as, for example, signalling over the trunk line or SS No. 7 signal digital data transmission, namely, in the case where the echo canceller must be disabled to satisfy a CCITT recommendation G.165, it needs additional tone detection and disabling circuits. This results in an increase in the cost per line.
The reliability of the echo canceller has an effect on the reliability of the trunk line because the echo canceller is built in the trunk line. Namely, if, since a particular echo canceller is used in a particular T1 or E1 trunk line, a fault occurs in the echo canceller, the corresponding trunk line is not available, thereby making speech impossible. For this reason, degradation in the reliability of the echo canceller results in a falling-off of the reliability of the trunk line.